The present invention relates to a method for treating a gas by adsorption, of the type in which only two adsorbers are used, which each follow the same, but shifted, cycle successively consisting of an adsorption phase, at a high cycle pressure, and of a regeneration phase with depressurization, which ends in the adsorber being repressurized, the method comprising a step of paralleling the adsorbers during which the total gas stream treated is obtained both by treating a first gas flow by at least one adsorber ending its adsorption phase, and treating a second gas flow to be treated by at least one other adsorber starting its adsorption phase.
The invention is especially applicable to the purification of air for distillation.
Such treatment methods are common in order to obtain, for example, a gas free from one or more constituents, considered as impurities with regard to the downstream treatments for which this treated gas is destined. They are implemented by treatment plants comprising at least two adsorbers. Thus air, for example, in order to be able to be distilled, must be thoroughly purified of water and of CO2. Generally, an air distillation unit is fed by a single air stream. This main air stream is purified at a single pressure level, this taking place in at least two bottles of adsorbents, one of which is in adsorption mode, while the other is in regeneration mode.
During the cycle of these two bottles, the two bottles are paralleled in the adsorption phase so as to provide continuity of air going toward the distillation unit. However, it is important to note that, when the bottle which has just been regenerated is returned to adsorption mode in order to produce purified air, a heat peak appears in the purified air at the outlet of this bottle. It is accepted that this temperature increase is due to the adsorption of nitrogen by the adsorber, and is all the more marked the higher the pressure to which the adsorber is subjected. This temperature variation adversely affects the behavior of the elements which are downstream, whether this is the main exchanger of the air distillation unit or a supercharger. In particular, this problem must be solved in the methods referred to as xe2x80x9cpumpxe2x80x9d methods where pressurized liquid oxygen is vaporized by liquefaction of high-pressure air.
EP-A-0146646 and U.S. Pat. No. 4,981,499 show methods of treating gas by adsorption using at least four adsorbers.
One of the aims of the invention is to limit the harmful consequences of this heat peak by proposing a method which is easy to implement and involving few structural modifications of the relevant treatment plants of the prior art.
To this end, the subject of the invention is a method of the aforementioned type, characterized in that the paralleling step is maintained for a predetermined time interval which is very much greater than the switching time of the valves controlling the adsorbers establishing said step, and/or as long as the difference in temperature between that of the total stream and that of at least one of said flows is greater than a predetermined temperature threshold.
According to other features of the invention:
said predetermined time interval is at least equal to 1 minute;
said predetermined time interval is at least equal to 2 minutes;
the first flow rate is varied at least partially during the paralleling step;
the second flow rate is varied at least partially during the paralleling step;
the second flow rate is varied in an opposite manner to the variations of the first flow rate;
the gas is air and the treatment is a purification of this air from water and CO2;
an adsorber is repressurized by means of a gas repressurization mixture containing at least one gas from air which is optionally dry and/or decarbonated, and a fluid which is richer in oxygen than air;
for adsorption at a maximum pressure of about 7 bar, the paralleling step is held for at least 1 minute and/or with a temperature threshold of about 20xc2x0 C.;
for adsorption at a maximum pressure of about 7 bar, the paralleling step is held for at least 2 minutes and/or with a temperature threshold of about 20xc2x0 C.;
for adsorption at a maximum pressure of about 7 bar, the paralleling step is held for at least 2 minutes and/or with a temperature threshold of about 15xc2x0 C.;
for adsorption at a maximum pressure of about 7 bar, the paralleling step is held for at least 5 minutes and/or with a temperature threshold of about 15xc2x0 C.;
for adsorption at a maximum pressure of about 7 bar, the paralleling step is held for at least 5 minutes and/or with a temperature threshold of about 10xc2x0 C.;
for adsorption at a maximum pressure of about 7 bar, the paralleling step is held for at least 10 minutes and/or with a temperature threshold of about 10xc2x0 C.;
for adsorption at a pressure of between about 7 and 36 bar, especially between about 20 and 36 bar, the paralleling step is held for at least 2 minutes and/or with a temperature threshold of about 30xc2x0 C.;
for adsorption at a pressure of between about 7 and 36 bar, especially between about 20 and 36 bar, the paralleling step is held for at least 5 minutes and/or with a temperature threshold of about 30xc2x0 C.;
for adsorption at a pressure of between about 7 and 36 bar, especially between about 20 and 36 bar, the paralleling step is held for at least 5 minutes and/or with a temperature threshold of about 25xc2x0 C.;
for adsorption at a pressure of between about 7 and 36 bar, especially between about 20 and 36 bar, the paralleling step is held for at least 10 minutes and/or with a temperature threshold of about 25xc2x0 C.;
for adsorption at a pressure of between about 7 and 36 bar, especially between about 20 and 36 bar, the paralleling step is held for at least 10 minutes and/or with a temperature threshold of about 20xc2x0 C.;
for adsorption at a pressure of between about 7 and 36 bar, especially between about 20 and 36 bar, the paralleling step is held for at least 15 minutes and/or with a temperature threshold of about 15xc2x0 C.;
the adsorbers follow the same phase-shifted cycle of time interval T with an adsorption phase having a time interval of between T/2 (inclusive) and T (not inclusive).
The subject of the invention is also a plant for treating a gas by adsorption in order to implement the method as defined above, of the type comprising a gas feed line; a treated gas discharge line; just two adsorbers which each follow the same, but shifted, cycle successively consisting of an adsorption phase, at a high cycle pressure, and of a regeneration phase with depressurization, which ends in the adsorber being repressurized, the method comprising a step of paralleling the adsorbers during which the total gas stream treated is obtained both by treating a first gas flow to be treated by at least one adsorber ending its adsorption phase, and treating a second gas flow to be treated by at least one other adsorber starting its adsorption phase; and means for connecting the adsorbers to the feed line, to the discharge line, to each other, and possibly to a line routing an oxygen-rich fluid, characterized in that the plant comprises a flow rate regulating device actuating the connection means such that said connection means hold the paralleling step for a predetermined time interval very much longer than the switching time of the valves controlling the adsorbers establishing said step, and/or as long as the temperature difference between that of the total flow and that of at least one of said flows is greater than a predetermined temperature threshold.
According to another feature of the plant, the regulating device comprises a differential temperature sensor for sensing the temperature difference between the total stream and the first flow, and/or between the total stream and the second flow.
Each adsorber comprises a bottle, each one containing either just a molecular sieve or a molecular sieve with alumina, capable of adsorbing the water and CO2 contained in the air.